1. Field of the Invention
This invention relates to autodeposition. Autodeposition involves the use of an aqueous resinous coating composition of relatively low solids concentration (usually less than about 10%) to form a coating of relatively high solids concentration (usually greater than about 10%) on a metallic surface immersed therein, with the coating increasing in thickness and areal density (mass per unit area of coating) the longer the time the metallic surface is immersed in the composition. Autodeposition is somewhat similar to electrodeposition but does not require the aid of external electrical current to cause the resin particles to deposit on the metal surface.
In general, autodepositing compositions are aqueous acid solutions having solid resin particles dispersed therein in very finely divided form. The coating formed while the metal substrate used is immersed in the bath is generally wet and fairly weak, although sufficiently strong to maintain itself against gravity and moderate spraying forces. In this state the coating is described as "uncured". To make an autodeposition coated object suitable for normal practical use, the uncured coated is dried, usually with the aid of heat. The coating is then described as "cured".
The present invention relates more particularly to the chemical treatment of an uncured autodeposited coating for the purpose of improving various properties thereof, particularly the adhesion of the coating to the underlying metal substrate and the resistance to corrosion of the underlying metal provided by the cured autodeposited coating when the coated metal surfaced object is subjected to corrosive environments.
2. Statement of Related Art
Basic constituents of an autodepositing composition are water, resin solids dispersed in the aqueous medium of the composition, and activator, that is, an ingredient or ingredients which convert the composition into one which will form on a metallic surface a resinous coating which increases in thickness or areal density as long as the surface is immersed in the composition. Various types of activators or activating systems are known, for example, as reported in the following U. S. Pat. Nos.: 3,592,699; 3,709,743; 4,103,049; 4,347,172; and 4,373,050, the disclosures of Which, to the extent not inconsistent with any explicit statement herein, are hereby incorporated herein by reference. The activating system generally comprises an acidic oxidizing system, for example: hydrogen peroxide and HF; HNO.sub.3 ; a ferric-containing compound and HF; and other soluble metal-containing compounds, for example, silver fluoride, ferrous oxide, cupric sulfate, cobaltous nitrate, silver acetate, ferrous phosphate, chromium fluoride, cadmium fluoride, stannous fluoride, lead dioxide, and silver nitrate in an amount between about 0.025 and about 50 grams per liter ("g/l") and an acid, which can be used alone or in combination with hydrofluoric acid, and including, for example, sulfuric, hydrochloric, nitric, and phosphoric acid, and organic acids, including, for example, acetic, chloroacetic, and trichloroacetic acids.
Previously known autodepositing compositions can be used to form coatings which have good aesthetic properties and which protect the underlying metallic substrate from being degraded (for example, corroded by water). However, there are certain applications which require that the autodeposited coating have particularly good properties for satisfactory use. Various means have been developed to improve the properties of autodeposited coatings, including, for example: chemical pretreatment of the metallic surface prior to formation of the coating; selection of particular resins for use in forming the coating; addition to the autodepositing composition of chemical additives; and chemical treatment of the freshly formed or uncured coating, as described in detail in copending application Serial No. 202,117 filed Jun. 3, 1988 and assigned to the same assignee as this application.
There are several U.S. patents which disclose the treatment of freshly formed autodeposited coatings with acidic aqueous solutions of one or more chromium compounds to improve the corrosion-resistance and/or surface appearance of the cured coating. Among such patents are Nos: 3,795,546; 4,030,945; 4,411,950; and 4,637,839, all assigned to the same assignee as that of the present invention. The '546 and '945 patents disclose treating an uncured autodeposited coating with an acidic aqueous solution containing hexavalent chromium or hexavalent chromium and formaldehyde-reduced forms of hexavalent chromium to improve the corrosion-resistant properties of the cured form of the coating and to reduce the gloss of an otherwise glossy coating. According to these patents, the source of chromium can be chromium trioxide or water-soluble salts of chromium or dichromate, for example, sodium, potassium, and lithium salts thereof. Optional ingredients of such chromium-containing solutions include phosphoric acid (anti-gelling agent), sodium hydroxide (pH adjuster), and a water-soluble or water-dispersible polyacrylic acid corrosion-resistant and paint-bonder improver). The '950 patent discloses the treatment of an uncured autodeposited coating with an aqueous chromium-containing solution which has dispersed therein particles of a resin which functions to impart to the cured form of the coating a reduced coefficient of friction. The patent discloses that the function of the chromium is to improve the corrosion-resistant properties of the cured coating, and the function of the resin, for example, polytetrafluoroethylene, is to increase the surface slip of the cured form of the coating. The '839 patent discloses the treatment of an uncured autodeposited coating with an acidic aqueous treating solution prepared by admixing a hexavalent chromium-containing compound (for example, ammonium and an alkali metal dichromate) with a hexavalent chromium/reduced chromium solution. In addition, the treating solution contains an acid or salt thereof, for example, hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, and ammonium, alkali metal, and alkaline earth metal salts of phosphoric acid. This patent discloses that the use of such a solution imparts a matte appearance to an autodeposited coating which otherwise would have a glossy appearance and improves the corrosion-resistant properties of the coating. In addition, U.S. Pat. No. 3,647,567 discloses the use of an acidic aqueous solution of chromium trioxide or of water-soluble or acid-soluble chromates and dichromates to improve the corrosion resistance of the resinous coatings described therein. Exemplary chromates and dichromates are sodium, ammonium, lithium, magnesium, potassium and zinc.
Japanese Patent No. 7630247 discloses the treatment of an uncured autodeposited coating with an aqueous solution or dispersion of a vulcanizing agent (for example, a sulfur-containing compound) or of a vulcanizing accelerator (for example, hexamethylenetetramine) to improve the solvent resistance of the cured coating.
In Japanese Patent No. 7630246, it is disclosed that adhesion of the freshly formed or wet coating to the underlying metallic substrate can be improved by contacting the coating with an acidic aqueous solution of an inorganic or organic acid or of an oxidizing agent (for example, sodium permanganate). This in turn leads to the provision of cured coatings which have a more uniform and appealing appearance. In addition to the use of chromium compounds, aforementioned U.S. Patent No. 3,647,567 teaches the use of an aqueous solution of phosphoric acid to improve the corrosion resistance of the resinous coating described therein.
In addition, Japanese Patent No. 7630245 discloses the treatment of an uncured autodeposited coating with an aqueous composition containing a water-miscible coalescing agent comprising a compound having two or more oxygen-containing functional groups such as ester groups, hydroxy groups, carbonyl groups and ether linkages. Examples of such classes of compounds include alcohols, ketones, alcohol esters, ketone esters, ketone ethers, and ester ethers. This Japanese patent discloses that the treatment of uncured autodeposited coatings with such coalescing agents inhibits or deters the tendency of the cured form of the coating to blister, crack and/or bridge.
It is an object of this invention to provide metallic surfaces, particularly surfaces that are made of one of the types of high carbon steel conventionally used for heavy duty springs and/or ferriferous surfaces that have been cold worked, especially by shot peening, grit blasting, or the like before being coated, with autodeposited coatings with better adhesion and/or better corrosion resistance than those obtained by following the teachings of the prior art.